VARIATIONS : HOW INDUCED 313 



barley or wheat with weak stems, by selecting and propagating 

 an individual plant with rigid straw, unless such stiffness is trans- 

 mitted to the descendants of the selected plant. 



Which variations exhibited by plants are transmitted to their 

 seedling offspring, and which are not, can only be determined by 

 trial. The variations of plants and animals must arise from 

 specific changes in the constitution of their protoplasm, but nothing 

 definite is known of the nature of these changes, and to cause 

 a plant to vary with certainty in some particular and desirable 

 manner is at present impossible. Even to make a plant vary at 

 all appreciably is often a matter of great difficulty, some species 

 being very stable. However, when variation once begins, the 

 desired character will sooner or later appear among the plants, 

 so that the first step in plant improvement is to ' break the type,' 

 or to make the type it is intended to improve vary in any manner 

 whatever. 



Since the variations of plants are the starting points from which 

 improvement or modification begins, it becomes important to 

 enquire if there are any methods by which variation can be in- 

 duced. 



Experience has taught that variation can be induced — 

 (i) by varying the external conditions of life of the plant ; 

 (2) by crossing and hybridisation. 



It is well known that an abundance of manurial constituents 

 leads to luxuriance of the various organs of a plant, while a re- 

 duction of these substances results in lowness of stature, and 

 general diminution of all parts ; poverty or richness of soil, there- 

 fore, leads to variation among plants. Similarly, the intensity of 

 the light, the warmth or coldness of the summer induces variations 

 in sweetness of almost all kinds of fruit. The size of the grains of 

 wheat, barley, and other cereals, and that of many seeds and other 

 parts of plants, is also dependent on the cultivation of the ground 

 in which they are grown, and upon the season and the length of 

 time during which growth goes on : other external conditions 



